You can’t without identifying the fungus and that is not always easy. Literally thousands of species of fungus occur in Britain; only a few are really good to eat and only a few are deadly poisonous.
The old wives’ tales that only edible fungi have a peelable ‘skin’, do not blacken a silver spoon (!) when cooking and smell like mushrooms, are all totally wrong!
Most fungus fatalities in Britain (about one each year) are caused by the death cap which has white gills, a ring (or ‘collar’) round the stem and, usually, a greenish cap.
Picture by Cheryl Dye